Methods for the preparation of lanthanum hexaboride, europium hexaboride and europium yttrium hexaboride crystals are known. The crystals have been used as cathodes for thermionic emission and field emission devices. The crystals have been produced basically by two methods.
A hot pressing technique described by Kawabe et al in U.S. pat. No. 3,932,314 to form (Y.sub.1-x Eu.sub.x)B.sub.6 comprises, inter alia, mixing the oxides Y.sub.2 O.sub.3 and Eu.sub.2 O.sub.3 with boron powder and polyvinyl alcohol as a molding agent and thereafter applying a pressure of about 1 ton/cm.sup.2 to compact the powders with subsequent heating in a boron nitride crucible at a temperature of about 1525.degree. C. for 1 hour in a vacuum of 10.sup.-2 to about 10.sup.-3 Torr.
Tanaka et al, J. Crystal Growth, 40 (1977), pp. 125-128, describe the formation of EuB.sub.6 by a borothermal reduction according to the following equation: EQU Eu.sub.2 O.sub.3 +15B.fwdarw.2EuB.sub.6 +3BO
The borothermal formation of EuB.sub.6 is similar to the work of Kawabe et al in that it involves the reaction of Eu.sub.2 O.sub.3 with boron powder, high pressure compressing and subsequent heating. The hot pressing technique tends to form polycrystalline materials which may incorporate binder impurities into the crystal which can degrade the performance of the hexaboride material.
U.S. Pat. No. 4,054,946 describes the formation of LaB.sub.6 single crystals by an aluminum flux technique. High purity lanthanum metal, boron powder and high purity aluminum are mixed together and heated to from about 1200.degree. to about 1600.degree. C. from a time period which may vary from several minutes to several days to produce LaB.sub.6 which precipitates from the aluminum flux as a single crystal compound upon cooling of the mixture. Thereafter the aluminum flux is dissolved with a strong acid such as HCl. Futamoto et al, Japan J. Appl. Phys., Vol. 14, No. 9, 1975, and Aita et al, Japan J. Appl. Phys., Vol. 13, pp. 1263-1266, No. 2, (1974), teach similar aluminum flux techniques.
The aluminum flux technique permits the production of single crystal hexaborides. However, the reactants involved, namely lanthanum or other lanthanide metals, tend to be spontaneously combustible when exposed to oxygen and therefore, the reactions must be carried out under an inert atmosphere. The extra care and precautions which must be taken when growing single crystal hexaborides by the above technique greatly add to the cost of production.
Thus, it would be highly desirable to have a technique which can produce, binary, ternary and quaternary single crystal metal hexaborides without the impurities associated with hot pressing or the expensive precautions necessary to prevent explosions during the crystal growing process.